I have found that providing estimates within a range is the most reasonable approach. The range is usually between "this is the likely amount of time with ever

I have found that providing estimates within a range is the most reasonable approach. The range is usually between "this is the likely amount of time with everything going as it should" and "this is the worst case scenario time with all that can go wrong". It's a pretty broad definition between the two but I find it gives clients a sense of the scope of the project. The minimum time does include some padding to include all the usual likely overruns (testing, requirements gathering, etc.) but not too much. The worst case time better really be worst case as I provide clients the expectation that it should not take longer than this. I find that usually I will end up somewhere in the middle of the estimate and the client is still happy since they didn't hit the worst case scenario. I find that this usually tends to be relatively effective even if as a rule of thumb the worst case time is 3-4x the minimum time. Of course, there is still a lot of analysis done to come up with this range and there is a lot of discussion with the client to help them understand the details on why and how it could go. At the end of the day, communication is king. I find that educating clients about the complexities of the process always helps to gain trust.